Various types of pointing devices are used as input devices that allow a user to manipulate a cursor of a computer. In laptop computers, a capacitive touch pad is commonly used to manipulate the cursor. For desktop computers, the ubiquitous computer mouse constitutes the large majority of pointing devices. The mouse allows for cursor manipulation on a visual display screen of a personal computer or workstation, for example. Cursor manipulation includes actions such as rapid relocation of a cursor from one area of the display screen to another area or selecting an object on a display screen.
In a prior art “roller ball” mouse, a user controls the cursor by moving the mouse over a surface, such as a mouse pad, so that the cursor moves on the display screen in a direction and a distance that is proportional to the movement of the mouse. The roller ball mouse, however, has drawbacks associated with many other devices that have mechanical parts. Namely, over time the mechanical components wear out, become dirty, or simply break down so that the cursor can no longer be accurately manipulated, if at all.
Recently, “optical” mice that use image sensors have been developed. For example, U.S. Pat. No. 6,281,882 describes an optical mouse that produces a collimated beam of light that illuminates a surface upon which the mouse rests. A sensor array portion of an image sensor receives the reflected images of the surface as the mouse is traversed across a surface. The changes in successive reflected images are then determined to be mouse movement, which is then interpreted as a desired cursor movement.
The '882 patent provides an excellent primer on optical mouse technology, detailing the physical construction of an optical mouse, the generation of interrogating light, the optical lens system for capturing the reflected light to form image frames, and the methods by which movement is determined. The present invention is concerned with the method by a “movement vector” is determined from the image frames. The term movement vector as used herein describes the magnitude and direction of the movement of the mouse. The movement vector, when provided to an associated computing device, can then be used to manipulate a cursor.
The '882 patent refers to several prior related patents, including U.S. Pat. No. 5,578,813, U.S. Pat. No. 5,644,139, and U.S. Pat. No. 5,786,804, which detail various methods the movement vector can be determined. In general, consecutive image frames are compared to one another to determine the relative motion of the mouse. The sensor array produces a two-dimensional image of the surface, the image being formed from discrete pixels. In the method described in the '139 patent, a first frame is stored. A second subsequent frame is then captured and stored. The second frame is compared to the first frame to determine the movement vector. Specifically, a correlation value is determined for each possible shift of the subsequent frame in the direction of each immediately neighboring pixel. For two-dimensional arrays, this corresponds to eight correlation calculations. Further, a correlation calculation is performed accounting for no shifting of the subsequent image. Thus, nine total correlation calculations are performed, relative to the first frame. The shift that results in the highest correlation provides the movement vector. Further detail on this technique can be found in the '139 patent.
However, because the correlation calculation is performed with a single pixel shift in each direction, the maximum speed by which the optical mouse can be moved (while still providing a usable signal) is limited. Specifically, the optical mouse cannot be moved translate more than one pixel distance between successively captured images without significant increase in the amount of correlation calculations. Thus, if the optical mouse is translated between one to two pixels between successive images, then 25 correlation calculations must be performed. If the optical mouse is translated between two to three pixels between successive images, 49 correlation calculations must be performed.